Redundancy and load balancing in a telecommunication unit and system

ABSTRACT

The invention relates to backing up a network element (NE) in a telecommunications system. The network element (NE) comprises at least two cluster nodes (A, B, C) that are redundancy units of each other. Each cluster node (A, B, C) contains virtual nodes (a 1 , a 2 , b 1 , b 2 , c 1 , c 2 ). Pairs are formed of the virtual nodes (a 1 , a 2 , b 1 , b 2 , c 1 , c 2 ) in such a manner that the first virtual node of the pair resides in the first cluster node and the second virtual node in the second cluster node. One of the virtual nodes in the pair is active and the other on standby. When a cluster node malfunctions, the virtual nodes of the pairs whose active virtual nodes reside in the faulty cluster node are interchanged by changing the standby virtual nodes to active and the active virtual nodes to standby.

BACKGROUND OF THE INVENTION

[0001] The invention relates to the redundancy of network elements andto load balancing in a telecommunications system, and especially tousing parallel gateway nodes, such as GGSNs (Gateway GPRS support node)in a packet-switched mobile system. To provide a concrete example, theinvention will be described in the context of a packet-switched mobilecommunication system.

[0002] The continuous development of applications transmitted in mobilesystems sets ever increasing demands on mobile networks. An efficientuse of the radio network determining the capacity of the system isimportant to enable extensive traffic. Packet-switched connections aremore efficient than circuit-switched connections in many applications.They are especially well suited for burst data transmission, such as forthe use of the Internet. A high bit rate is then required to load a newpage, but, on the other hand, data traffic is almost non-existent whenthe page is viewed. In circuit-switched connections, the capacity of theconnection is, however, all the time reserved for a certain user,whereby resources are wasted and the user must also pay for this. In apacket-switched system, resource allocation is based on the amount oftransmitted data and not the duration of the connection.

[0003] GPRS (General packet radio service) is a technique enablingpacket-switched data transmission that will be utilized in thethird-generation mobile network UMTS (Universal mobiletelecommunications system), for instance. GPRS requires the introductionof new network elements, such as GGSN, in the mobile system. GGSN is thenetwork element of the GPRS and UMTS mobile networks and controls therouting of data packets in the GPRS network and takes care of connectingthe GPRS network to other networks, such as the Internet and other GPRSnetworks.

[0004] In the GPRS system, the logical connection between a mobilestation and GGSN supporting the mobile station is called a PDP (Packetdata protocol) context. A redundant GGSN node comprises several GTP—U(GPRS tunnelling protocol—User plane) and GTP—C (GPRS tunnellingprotocol—Control plane) processing units that apply packet transmissionbased on PDP contexts. Redundancy is used in the GTP—U and GTP—Cprocessing units to continue transmitting packets even in errorsituations. The redundancy is based on having a second processing unittake over, if the primary unit cannot continue transmitting packets. Theredundancy of network nodes, such as GGSN, is typically implementedusing backup units with a redundancy ratio of 1:1, whereby there is onebackup unit for each active unit. The problem with 1:1 redundancy isthat it makes the structure of the network node heavy and expensive, ifevery processing unit is to be backed up.

BRIEF DESCRIPTION OF THE INVENTION

[0005] It is thus an object of the invention to solve the problem. It isan object of the invention to lower the hardware overhead to obtainredundancy. The object is achieved by developing a method and a systemimplementing the method and a network element that are characterized bywhat is stated in the independent claims. Preferred embodiments of theinvention are disclosed in the dependent claims.

[0006] The invention is based on using clusters, comprising parallelnetwork element units, called cluster nodes, for backing up a networkelement, such as GGSN. A cluster node is an example of a GTP—U or aGTP—C processing unit capable of serving PDP context activationrequests. Processing units serve as backup units for each other. Thecluster nodes include logical nodes that represent the pairs formed ofthe cluster nodes in such a manner that a pair of logical nodes isassociated with each pair, and one of the logical nodes resides in thefirst cluster node and the other in the second cluster node. In thelogical node pair, one of the logical nodes is active and the other ison standby. A directed logical node pair, which indicates the active andstandby logical node, is referred to as a load allocation alternative.

[0007] GGSN redundancy is then based on the idea that when a user planenode malfunctions, the PDP contexts whose active logical node resides inthe faulty cluster node will be served by the standby logical node ofthe pair, which thereafter becomes the active logical node.

[0008] The method, system and network element of the invention providethe advantage that 1:1 redundancy is not needed in the system and a paircan be defined for each cluster node even if there is an odd number ofcluster nodes, for instance three. This way, when one cluster nodemalfunctions, only 33% of the PDP contexts need to be transferred to beserved by another clus-ter node assuming that the load of the networkelement is divided between three cluster nodes.

[0009] In one embodiment of the invention, the network load is balancedin such a manner that when activating a PDP context, a logical node canbe selected as the active node from the cluster node that has the leastload. This embodiment also provides the advantage that one solution isprovided for both network element load balancing and unit redundancy.This way, the system becomes resilient, i.e. has high availability andreliability. This is advantageous especially in a situation in whichonly a part of the sessions must have high availability. This embodimentis especially well suited for an environment com-prising severalall-IP-GGSN (Internet protocol) units based on solely packet-switcheddata transmission, in which resiliency and availability are required ofthe system. A further advantage of this embodiment is that it alleviatesthe problem related to load balancing based on IP-based packettransmission, i.e. the fact that the transmission rate can decreasesignificantly when the load increases.

[0010] The invention also provides a combined resiliency solution forthe GTP—C and GTP—U processing units.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will now be described in greater detail by means ofthe preferred embodiments and with reference to the attached drawings,in which

[0012]FIG. 1 is a block diagram of a simplified system of the invention,

[0013]FIG. 2 is a schematic representation of external IP addressing ofthe invention on the user plane,

[0014]FIG. 3 is a schematic representation of load balancing of theinvention on the user plane, when a high-speed internal switch isavailable,

[0015]FIG. 4 is a schematic representation of load balancing of theinvention on the user plane, when a high-speed internal switch is notavailable,

[0016]FIG. 5 is a schematic representation of external IP addressing ofthe invention on the user plane and control plane,

[0017]FIG. 6 is a schematic representation of load balancing of theinvention on the user plane and control plane, when a high-speedinternal switch is available,

[0018]FIG. 7 is a schematic representation of load balancing of theinvention on the user plane and control plane, when a high-speedinternal switch is not available.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention can be applied to any telecommunicationssystem in which network element redundancy is implemented using,together with active units, standby units that can be activated when theactive unit malfunctions. These systems include third-generation mobilesystems, such as UMTS (Universal mobile telecommunications system), andsystems based on them, and the systems, such as GSM 1800 and PCS(Personal communications system), corresponding to the GSM system(Global system for mobile communications). The invention can also beapplied to other wireless systems and fixed systems.

[0020] In the following, the invention is described using an examplesystem that is based on a 3GPP-all-IP system, without restricting theinvention thereto, however. 3GPP-all-IP is an IP technology-based systemutilizing GPRS defined in the Third-generation partnership project 3GPP,in which system network element redundancy is implemented using parallelbackup units.

[0021]FIG. 1 shows a simplified GPRS architecture that only shows theparts that are essential for understanding the invention. It is apparentto a person skilled in the art that a mobile system also comprises otherfunctions and structures that need not be described in detail herein.

[0022] The main parts of a mobile system are a core network CN, a radioaccess network RAN and a mobile station, also referred to as userequipment UE. The GPRS system uses a 3G radio access network (such asthe UMTS radio access network) or a 2G radio access network (such as theGSM radio access network).

[0023] The GPRS core network CN can be connected to external networks,such as the Internet. The main parts of the core network CN are aserving gateway support node SGSN and a gateway GPRS support node GGSN.The core network described herein is based on the UMTS core network.Other types of core networks, for instance IS-41, can comprise othernetwork elements.

[0024] The main functions of SGSN are detecting new GPRS mobile stationsUE in its service area, processing the registrations of the new mobilestations UE, transmitting data packets to and from a GPRS mobile stationUE and maintaining a register on the locations of the mobile stations inthe service area.

[0025] The main functions of GGSN include interaction with an externaldata network. GGSN connects the GPRS network of the operator to externalsystems, such as the GPRS systems of other operators, and data networks,such as the Internet. GGSN contains the PDP addresses of the GPRSsubscribers and routing information, in other words, the SGSN addresses.The SGSN-side interface of GGSN is called the Gn interface and the IPnetwork-side interface is called the Gi interface. The interface betweenSGSN and a network managed by another network operator PLMN2 (Publicland mobile network) is called the Gp interface. The operation of GGSNof the invention will be described later in connection with FIGS. 2, 3,4, 5 and 6.

[0026] The mobile station UE can be a speech-only terminal or it can bea multi-service terminal that serves as a service platform and supportsthe loading and execution of different functions related to services.The mobile station UE comprises actual mobile equipment and, typically,a detachably attached identification card, also called a subscriberidentity module, SIM. The mobile station can be any device or acombination of several different devices capable of communicating in amobile system. The subscriber identity module typically includes thesubscriber's identifier, executes authentication algorithms, storesauthentication and encryption keys and subscriber information requiredin the mobile station.

[0027] To transmit and receive GPRS data, the mobile station UE mustactivate at least one PDP context that it wants to use. PDP refers to aprotocol transmitting data in packets. This activation makes the mobilestation known to the corresponding GGSN, and interworking with thenetwork can begin. A PDP context defines different data transmissionparameters, such as the PDP address, quality of service QoS and NSAPI(Network service access point identifier).

[0028] A mobile station connected to a GPRS system can at any time startPDP context activation by transmitting an activate PDP context requestto SGSN. After receiving the message, SGSN transmits a create PDPcontext request to GGSN that creates the PDP context and transmits it toSGSN. SGSN transmits an activate PDP context acknowledgement to themobile station UE in response to a successful PDP context activation,after which a virtual connection is established between the mobilestation UE and GGSN. As a result, SGSN transmits data packets comingfrom the mobile station UE to GGSN and GGSN transmits data packetsreceived from an external network through SGSN to the mobile station UE.The PDP context is stored in the mobile station UE, SGSN and GGSN.Without restricting it to the GPRS system, the PDP context is anylogical context that is established for the transmission ofpacket-switched data between a terminal and the network elementcontrolling the connection. One or more PDP contexts represent each PDPaddress in the mobile station UE, SGSN and GGSN.

[0029] GTP (GPRS tunnelling protocol) refers to a protocol that is usedto transmit user data between GPRS nodes in the GPRS core network. TwoPDP contexts on different interfaces and connected to each other form aGTP tunnel.

[0030] A system implementing the functionality of the present inventionand its network element comprise not only prior-art means but also meansfor implementing the functions described in more detail in connectionwith FIGS. 2, 3, 4, 5, 6 or 7. To be more specific, they comprisemaintenance means for maintaining virtual cluster nodes in the clusternodes, means for forming load allocation alternatives of said virtualcluster nodes and/or means for changing the load allocation. Inaddition, the network nodes can comprise means for distributing the loadto active cluster nodes, means for distributing the load of the networkelement between the cluster nodes, means for distributing the load ofthe network element on the basis of a load allocation plan, means fordefining an external IP address for the load allocation alternatives,means for maintaining information on a primary and secondary clusternode associated with the load allocation alternative, switching meansfor transmitting data by using the IP address defined for the loadallocation alternative and/or means for changing the load allocationinternally in the network element. It is also possible that the systemand its network nodes comprise all the above-mentioned means.

[0031] The present network nodes comprise processors and memory that canbe utilized in the functions of the invention. All alterations requiredto implement the invention can be performed as added and updatedsoftware routines and/or using hardware-based solutions, such as ASIC(application-specific integrated circuit) circuits, or a separate logic.

[0032] In the following description, the terms ‘control plane’ (CP) and‘user plane’ (UP) are used. All information transmitted and received bya mobile station user, such as coded voice data in voice calls orpackets of an Internet connection, are transmitted on the user plane.The control plane is used for all control signalling related to themobile system that is usually not visible to the user. There may beexceptions to this, for instance short messages can be transmitted onthe control plane. On the radio interface, the control-plane anduser-plane data can be multiplexed to the same physical channel.

[0033] Implementation of the First Embodiment

[0034] The following describes, how the invention is implemented on theuser plane.

[0035]FIG. 2 shows a network element NE, which is for instance a GGSNnode. The network element comprises two or more physical GTP—Uprocessing units A, B, C, which are herein also called cluster nodes.The cluster nodes A, B, C are capable of serving PDP contexts. Thecluster nodes are arranged in pairs such that each cluster node can forma pair with every other cluster node. For instance, if the number ofavailable cluster nodes N=3, the number of possible pairs P=3. If N=4,then P=6. In the situation shown in Figure 2, the number of clusternodes N=3, and the possible cluster node pairs are then AB, BC and CA.In each pair, the first cluster node is a backup unit of the secondcluster node.

[0036] According to the present invention, the cluster nodes containlogical nodes a1, a2, b1, b2, c1, c2, which are also called virtualcluster nodes or virtual nodes. The number of virtual nodes ispreferably twice the number of the pairs formed of the cluster nodes.The virtual cluster nodes a1, a2, b1, b2, c1, c2 are logical GTP—Uprocessing units. They are arranged in pairs in such a manner that thefirst virtual node of the pair is active and the second is on standby.The same cluster node can comprise both active and standby virtualnodes.

[0037] A directed virtual node pair has a feature visible outside thenetwork element called a load allocation alternative LBX1, LBX2, LBY1,LBY2, LBZ1, LBZ2, which is a logical Gn, Gp or Gi interface. Table 1shows the load allocation alternatives for three processing units A, B,C, and illustrates which of the virtual nodes of the load allocationalternative is active and which is on standby. TABLE 1 Virtual nodeProcessing unit pair Load allocation alternative Active Standby AB LBX1a1 b2 LBX2 b2 a1 BC LBY1 b1 c2 LBY2 c2 b1 CA LBZ1 c1 a2 LBZ2 a2 c1

[0038] Two or more virtual node pairs formed in an equal manner can havedifferent load allocation alternatives which may have other differencesapart from the formulation of the virtual node pair.

[0039] Each load allocation alternative has an external IP address thatis used as the user plane address of the PDP context. When a PDP contextactivation request is processed in GGSN, one of the load allocationalternatives is selected. A serving virtual node, i.e. active node, anda standby virtual node is selected. The selection is based for instanceon the load information of the cluster nodes or a specific alternatingdiagram.

[0040] If a cluster node A, B, C malfunctions, allocation of the PDPcontexts, whose active unit this cluster node is, is changed. The activevirtual node serving the PDP context is put on standby and thecorresponding standby virtual node becomes active, unless it happens tobe faulty as well. In this description, the change of the active andstandby unit is also referred to as a ‘switchover’. When thecorresponding standby units become active, they start serving the PDPcontexts.

[0041] Each load allocation alternative has an individual external userplane IP address at the Gn or Gp interface for receiving the datapackets that arrive at GGSN. This individual address of the loadallocation alternative is used as the PDP context address of the activevirtual node of the load allocation alternative. It is the feature ofthe load allocation alternative that is visible at the externalinterfaces of the network element. The IP address is used to indicatethe route through the physical interface of the cluster node A, B, C.

[0042] The traffic in the network element NE may be distributed betweenthe cluster nodes that comprise active virtual nodes on the basis of aspecific load allocation plan. The traffic in the network element NE maybe distributed between the cluster nodes that comprise standby virtualnodes, whereby the standby virtual nodes are made active.

[0043] Implementation of the First Embodiment Using an Internal Switch

[0044]FIG. 3 shows an implementation of the invention on the user plane,when GGSN has a high-speed internal switch K or a correspondingconnection between the cluster nodes A, B, C. By means of the internalswitch K, any alterations required to recover from faults can be hiddenat the interfaces of the network element, whereby the change of theactive cluster node to the standby node and vice versa is not visibleoutside the network element NE. Packets arriving at the physical Giinterface Gif or the Gn interface Gnf of the cluster node A, B, C aretransmitted through the internal switch K to the active cluster node.For the sake of clarity, FIG. 3 shows one switch K, but in reality,there may be several switches. Gnf could also illustrate the Gpinterface.

[0045] According to a preferred embodiment of the invention, loadallocation is based on routing (routing-based link resiliency)protocols, in other words, information on a primary and secondary routeto the load allocation alternative is maintained inside GGSN. Theprimary route is the physical interface of the cluster node comprisingthe active logical node of the load allocation alternative. Thesecondary route is the physical interface of the standby unit. Whenactivating a PDP context, the load allocation alternative is offeredfrom a cluster node A, B, C that has available capacity. The physical Gninterface Gnf or physical Gi interface Gif, at which the packets arrivein the network node, need not reside in the cluster node from which theload allocation alternative is offered. In other words, the packets canarrive at any interface and they are transmitted through the internalswitch K of the network node to the active cluster node. Forwardinginformation is maintained in GGSN to enable indicating the primary andsecondary route through the physical Gi or Gn interface of the clusternode to the active unit. The load allocation change or a switchoverbetween the active unit and the standby unit is not visible outside thenetwork element, because the external IP address of the load allocationalternative is the only address visible outside the network element.When the primary connection malfunctions, data packets are guided to usethe secondary route of the load allocation alternative. A fault in thefirst interface (for instance Gn) of GGSN is not visible to the secondinterface (for instance Gi).

[0046] According to another preferred embodiment of the invention, loadallocation is based on a link layer solution (link layer resiliency). Inthis case, too, the physical Gn interface Gnf or physical Gi interfaceGif need not reside in the cluster node A, B, C, in which the activeunit processes packets. In the link layer solution, a load allocationalternative has a physical interface dedicated for it, and the standbyunit monitors the physical connection or interface Gif, Gnf of theactive unit. Another GGSN component can also perform the task. If thestandby unit receives information on a malfunction of the interface usedby the active unit, the standby unit initiates the switchover. Thestandby unit then starts to use the backup interface instead of thededicated physical interface Gif, Gnf of the faulty unit, and packetsaddressed to the faulty unit are directed to the standby unit throughthe internal switch K. In this embodiment, too, a switchover or aninternal load allocation change of the load allocation alternative isnot visible outside the network element as a change in the external IPaddress but as a change in the address on the link layer, because the IPaddress of the load allocation alternative is the only routing addressvisible outside the network element. When the primary connectionmalfunctions, data packets are guided to use the secondary route of theload allocation alternative. A fault in the first interface (forinstance Gn) of GGSN is not visible to the second interface (forinstance Gi).

[0047] According to yet another preferred embodiment of the invention,the above-mentioned solution based on routing and the solution of thelink layer can be applied simultaneously, in which case first a quickrecovery from the malfunction of the cluster node takes place based onthe link layer solution and then a recovery based on the routing.

[0048] The visibility of a load allocation alternative may be on bothsides of the network element NE. This means that there may be a similarfeature of the load allocation alternative on the Gi and the Gn side ofGGSN. The external addressing of the load allocation alternatives may bebased for instance on the subnet address range used for the PDP contextIP addresses, the IP tunnel endpoint address in case of a tunnelingmechanism (such as Generic routing encapsulation GRE, IP-in-IP, or IPsecurity protocol IPSec), or a set of label switched paths used by theload allocation alternative.

[0049] Methods for guiding data packets to use a secondary route to theload allocation alternative in a routing-based solution or for startingthe use of an alternative physical interface in the link layer solutionare described later in ‘Changing of the physical interface’ of thisdescription.

[0050] Implementation of the First Embodiment Without an Internal Switch

[0051]FIG. 4 shows an implementation of the invention on the user plane,when no high-speed internal switch is available in the network element.The switchover or the load allocation change is then not an internalnetwork element change, but is visible outside. In other words, theroute changes on both interfaces Gn, Gi.

[0052] In yet another preferred embodiment of the invention, anintegrated load allocation change or switchover is performed. In arouting-based solution, data packets then arrive at the network elementNE in such a manner that the external address of the load allocationalternative is marked as their IP address. The primary routes fortransmitting data packets are the one that use the physical Gi interfaceGif or physical Gn interface Gnf of the cluster node A, B, C, in whichthe active load allocation alternative resides. When the cluster nodemalfunctions, the secondary routes are used. When using the secondaryroutes, the packets arrive at the physical interface of the clusternode, in which the second load allocation alternative resides and whichthen becomes the active unit. The change of the route to the secondaryroute is visible outside the network element. Routing protocols can beused to indicate the secondary route on the physical interface.

[0053] In an integrated load allocation change of yet another preferredembodiment of the invention, it is possible to use a link layer solutionthat is based on the idea that the standby unit of the load allocationalternative monitors the physical Gi interface Gif and physical Gninterface Gnf of the active unit. Another GGSN component can alsoperform this task. If an error is detected, the standby unit starts touse the alternative physical interface of the faulty unit. Methods forchanging the physical interface are described later in ‘Changing of thephysical interface’ of this description. The standby unit can then startto use the alternative physical interface, if it is the standby unit ofall the PDP contexts whose active unit the faulty unit is. This can beachieved by indicating routes on the physical interface of the standbyunit that replaced the faulty unit. Because the Gn-side changes alsoneed to be made on the Gi-side (and vice versa), a logical Gi interface(or Gn interface) is allocated for each load allocation alternative.

[0054] The routing solution or the link layer solution can be applied tothe Gi interface regardless of which solution is applied to the Gninterface, and vice versa. Methods for guiding data packets to use asecondary route to the load allocation alternative in a routing-basedsolution or for starting the use of the alternative physical interfacein the link layer solution are described later in ‘Changing of thephysical interface’ of this description.

[0055] Implementation of the Second Embodiment

[0056] The following describes a combined user plane and control planeimplementation of the invention.

[0057]FIG. 5 shows a network element NE, which is for instance a GGSNnode. The network element comprises two or more physical GTP—Uprocessing units A, B, C, which are herein also called user planecluster nodes, and two or more physical GTP—C processing units D, E, Fwhich are herein also called control plane cluster nodes. The user planeand control plane cluster nodes A, B, C, D, E, F are capable of servingPDP contexts. According to the second embodiment of the invention,serving pairs are formed of the user plane and control plane clusternodes such that a user plane cluster node and a control plane clusternode form a pair. In a serving pair, both of the nodes are active. Eachuser plane cluster node can form a pair with every control plane clusternode and vice versa. The serving pair has a backup pair on standby. Eachbackup pair can be the backup pair of every serving pair. The backuppair also comprises a control plane cluster node and a user planecluster node.

[0058] According to this embodiment, the user plane and control planecluster nodes comprise logical nodes, which are also called virtualcluster nodes or virtual nodes a4, b4, d4, e4. The virtual nodes arelogical GTP—U or GTP—C processing units. The virtual nodes are arrangedin pairs such that the first virtual node of the first pair resides inthe GTP—U processing unit of the serving pair and the second virtualnode of the first pair resides in the GTP—U processing unit of thebackup pair, and such that the first virtual node of the second pairresides in the GTP—C processing unit of the serving pair and the secondvirtual node of the second pair resides in the GTP—C processing unit ofthe backup pair. The virtual node pairs are further arranged insecondary pairs such that the first virtual node pair resides in theGTP—U processing units and the second virtual node pair resides in theGTP—C processing units. Table 2 shows the possible serving pairs andbackup pairs for three GTP—U processing units A, B, C and three GTP—Cprocessing units D, E, F. TABLE 2 UP processing units CP processingunits Serving pair + Backup pair A, B D, E AD + BE AE + BD BE + AD BD +AE A, B E, F AE + BF AF + BE BF + AE BE + AF A, B F, D AF + BD AD + BFBD + AF BF + AD B, C D, E BD + CE BE + CD CE + BD CD + BE B, C E, F BE +CF BF + CE CF + BE CE + BF B, C F, D BF + CD BD + CF CD + BF CF + BD C,A D, E CD + AE CE + AD AE + CD AD + CE C, A E, F CE + AF CF + AE AF + CEAE + CF C, A F, D CF + AD CD + AF AD + CF AF + CD

[0059] The control plane virtual node d4, e4 and the user plane virtualnode a4, b4 of the backup pair may reside in different physical unitseven if the virtual nodes of the serving pair resided in the samephysical unit and vice versa.

[0060] A directed secondary virtual node pair forms a load allocationalternative LB1. A directed secondary virtual node pair indicates whichare the active virtual nodes and standby virtual nodes associated withit. In the load allocation alternative LB1, user plane virtual node a4and control plane virtual node d4 are active, and user plane virtualnode b4 and control plane virtual node e4 are their standby virtualnodes.

[0061] Two or more secondary virtual node pairs formed in an equalmanner can have different load allocation alternatives which may haveother differences apart from the formulation of the secondary virtualnode pair.

[0062] Load allocation alternatives have external IP addresses that areused as the addresses of the PDP contexts. The address may be differentfor user plane and control plane. When an activate PDP context requestis processed in GGSN, load allocation alternatives for the user planeand control plane are selected. When selecting a load allocationalternative the (initial) serving pair and (initial) backup pair areselected. The IP address of the user plane, e.g. IP address for clusternodes A and B, and the IP address control plane, e.g. IP address forcluster nodes D and E, are selected. The selection is based for instanceon the load information of the cluster nodes or on a specificalternating diagram.

[0063] If malfunctioning of a cluster node prevents a virtual node tocontinue as the active node, the backup virtual node is made the activenode. This may be done separately on user plane and control plane. Aswitchover is thus performed. The active virtual node pair serving thePDP context is put on standby and the corresponding standby virtual nodepair becomes active, unless it happens to be in a faulty unit as well.When the corresponding standby units become active units they startserving the PDP contexts. Load allocation of the control plane clusternode does not necessarily have to be changed if the faulty unit is auser plane cluster node, and vice versa.

[0064] The routing address of the load allocation alternative is used asthe PDP context address of the active virtual node of the loadallocation alternative. It is the feature of the load allocationalternative that is visible outside the network element. The IP addressis used to indicate the route through the physical interface of thecluster node.

[0065] The traffic in the network element NE may be distributed betweenthe cluster nodes that comprise active virtual nodes on the basis of aspecific load allocation plan. The traffic in the network element NE maybe distributed between the cluster nodes that comprise standby virtualnodes, whereby the standby virtual nodes are made active.

[0066] Implementation of the Second Embodiment Using an Internal Switch

[0067]FIG. 6 shows the implementation of the invention on the userplane, when GGSN has a high-speed internal switch K or a correspondingconnection between the cluster nodes A, B, C, D, E, F. By means of theinternal switch K, any alterations required to recover from faults canbe hidden at the interfaces of the network element, whereby the changeof the active cluster node to the standby node and vice versa is notvisible outside the network element NE. Packets arriving at the physicalGi interface Gif or the Gn interface Gnf of the cluster node A, B, C aretransmitted through the internal switch K to the active cluster node.For the sake of clarity, FIG. 6 shows one switch K, but in reality,there may be several switches.

[0068] According to a preferred embodiment of the invention, loadallocation is based on routing protocols (routing based linkresiliency), in other words, information on a primary and secondaryroute to the load allocation alternative is maintained inside GGSN. Theprimary route is the physical interface of the cluster nodes comprisingthe serving pair. The secondary route is the physical interface of thecluster nodes comprising the backup pair. When activating a PDP context,the load allocation alternative is offered from a cluster node pair thathas capacity available. The physical Gn interface Gnf or physical Giinterface Gif, at which the packets arrive in the network node, need notreside in the cluster nodes from which the load allocation alternativeis offered. In other words, the packets can arrive at any interface andthey are transmitted through the internal switch K of the network nodeto the active units. Forwarding information is maintained in GGSN toenable indicating the primary and secondary route through the physicalGi or Gn interface of the cluster node to the active units. Theswitchover or internal load allocation change of a load allocationalternative is not visible outside the network element, because the IPaddresses of the load allocation alternative are the only addressesvisible outside the network element. When the primary connectionmalfunctions, data packets are guided to use the secondary route of theload allocation alternative. A fault in the first interface (forinstance Gn) of GGSN is not visible to the second interface (forinstance Gi).

[0069] According to yet another preferred embodiment of the invention,allocation of PDP contexts is based on a link layer solution (link layerresiliency). In this case, too, the physical Gn interface Gnf orphysical Gi interface Gif need not reside in the cluster node pair, inwhich the active units process packets. In the link layer solution, aload allocation alternative has a physical interface dedicated for it,and the standby units monitor the physical connection or interface Gif,Gnf of the active units. Another GGSN component can also perform thetask. If the standby units receive information on a malfunction of theinterface used by the active units, the standby units initiate theswitchover. The standby units then start to use the backup interfaceinstead of the dedicated physical interface Gif, Gnf of the faulty unit,and packets addressed to the faulty unit are directed to the standbyunits through the internal switch K. In this embodiment, the switchoveror internal load allocation change of the load allocation alternative isnot visible outside the network element as a change in the external IPaddress but as a change in the address on the link layer, because the IPaddress of the load allocation alternative is the routing addressvisible outside the network element. When the primary connectionmalfunctions, data packets are guided to use the secondary route of theload allocation alternative. A fault in the first interface (forinstance Gn) of GGSN is not visible to the second interface (forinstance Gi).

[0070] According to yet another preferred embodiment of the invention,the above-mentioned solution based on routing and the solution of thelink layer can be applied simultaneously, in which case first a quickrecovery from the malfunction of the cluster node takes place based onthe link layer solution and then a recovery based on the routing.

[0071] The visibility of a load allocation alternative may be on bothsides of the network element NE. This means that there may be a similarfeature of the load allocation alternative on the Gi and the Gn side ofGGSN. The external addressing of the load allocation alternatives may bebased for instance on the subnet address range used for the PDP contextIP addresses, the IP tunnel endpoint address in case of a tunnelingmechanism (such as GRE, IP-in-IP, or IPSec), or a set of label switchedpaths used by the load allocation alternative.

[0072] Methods for guiding data packets to use a secondary route to theload allocation alternative in a routing-based solution or for startingthe use of an alternative physical interface in the link layer solutionare described later in ‘Changing of the physical interface’ of thisdescription.

[0073] Implementation of the Second Embodiment Without an InternalSwitch

[0074]FIG. 7 shows an implementation of the invention on the user plane,when no high-speed internal switch is available in the network element.The load allocation change or a switchover is then not an internalnetwork element change, but is visible outside. In other words, theroute of the data packets changes on both interfaces Gn, Gi.

[0075] In yet another preferred embodiment of the invention, anintegrated load allocation change is performed. In a routing-basedsolution, data packets then arrive at the network element NE in such amanner that the external address of the load allocation alternative ismarked as their IP address. The primary route for transmitting datapackets is the one that uses the physical Gi interface Gif or physicalGn interface Gnf of the cluster nodes A, B, C, D, E, F in which theactive load allocation alternative resides. When the cluster nodemalfunctions, the secondary route is used. When using the secondaryroute, the packets arrive at the physical interface of the clusternodes, in which the backup pair resides and which then become the activeunits. The change of the route to the secondary route is visible outsidethe network element. Routing protocols can be used to indicate thesecondary route on the physical interface.

[0076] In an integrated load allocation change or a switchover of apreferred embodiment of the invention, it is possible to use a linklayer solution that is based on the idea that the standby units of theload allocation alternative monitor the physical Gi interface Gif andphysical Gn interface Gnf of the active units. Another GGSN componentcan also perform this task. If an error is detected, the standby unitstarts to use the alternative physical interface of the faulty unit.Methods for changing the physical interface are described later in‘Changing of the physical interface’ of this description. The standbyunit can then start to use the alternative physical interface, if it isthe standby unit of all the PDP contexts whose active unit the faultyunit is. This can be achieved by indicating routes on the physicalinterface of the standby unit that replaced the faulty unit. Because theGn-side changes also need to be made on the Gi-side (and vice versa), alogical Gi interface (or Gn interface) is allocated for each loadallocation alternative.

[0077] The routing solution or the link layer solution can be applied tothe Gi interface regardless of which solution is applied to the Gninterface, and vice versa. Methods for guiding data packets to use asecondary route to the load allocation alternative in a routing-basedsolution or for starting the use of the alternative physical interfacein the link layer solution are described next in ‘Changing of thephysical interface’ of this description.

[0078] Changing of the Physical Interface

[0079] When binding an IP address to a new link layer address in thesituations described above, data packets may be lost. Methods fortransmitting data arriving at the network element NE to the physical Gior Gn interface of the active cluster node A, B, C, D, E, F in thesituations shown in FIGS. 3, 4, 6 or 7 without a packet loss are theforwarding, unicast and multicast modes. Another benefit of thesemethods is that a dedicated physical interface is not needed.

[0080] The forwarding mode means that one of the cluster nodes serves asthe master to the group IP routing address of the network element or tothe external address(es) of the load allocation alternative. The masterreceives the packets addressed to the group IP routing address or to theexternal address of the load allocation alternative and forwards them tothe active cluster node. The forwarding is based for instance on theload allocation alternative address of the data packet. If the mastermalfunctions, another master is selected in the network element, towhich packets addressed to the group IP routing address are thereaftertransmitted. The forwarding mode can be applied to the routing-basedsolutions described earlier in such a manner that the network elementeither has or does not have an internal switch available to it.

[0081] The unicast mode means that data arriving at the network elementNE is transmitted separately to each cluster node, even though the dataonly has one receiver. Each cluster node then receives the packet andeither accepts the packet or rejects it depending on the routingaddress(es) of the load allocation alternative of the data in question.The rejection or the acceptance of the packet may be due the contents ofthe data packet as well.

[0082] The multicast mode means that data can have several simultaneousreceivers. The data packet then arrives at the group IP routing addressof the network element, from which it is forwarded on in multicasting toall cluster nodes. Depending on the routing address(es) of the loadallocation alternative of the data, the cluster node either accepts orrejects the packet. The rejection or the acceptance of the packet may bedue the contents of the data packet as well.

[0083] Even though the invention is described above with the GTP—U orGTP—C management in GGSN as an example, it is apparent to a personskilled in the art that the invention can also be applied to otherprotocols. The invention can also be applied to othercluster-implemented network elements. Examples of other networkelements, to which the invention can be applied, are a serving GPRS node(SGSN), an IP base transceiver station IP-BTS and a radio access networkgateway RAN-GW.

[0084] Even though the invention is presented above by describing theredundancy of both the input and output interface and the loadallocation of the network element, it is apparent to a person skilled inthe art that the invention can also be applied to situations, in whichonly one of the interfaces is used.

[0085] It is apparent to a person skilled in the art that while thetechnology advances, the basic idea of the invention can be implementedin many different ways. The invention and its embodiments are thus notrestricted to the examples described above, but can vary within thescope of the claims.

1. A method for backing up a network element in a communications system,the network element comprising at least a first and a second parallelphysical cluster node, which cluster nodes are capable of transmittingdata, whereby the first cluster node is a redundancy unit to the secondcluster node and vice versa, wherein the method comprises the steps of:maintaining one or more logical nodes in each of the first and secondcluster nodes, forming load allocation alternatives of the logical nodeswherein the first logical node of the load allocation alternativeresides in the first cluster node and the second logical node resides inthe second cluster node, whereby the first logical node is active andthe second logical node on standby or vice versa, and performing, when acluster node malfunctions, a switchover of the load allocationalternatives, the active logical nodes of which reside in the faultycluster node, by changing their logical nodes from standby to active andthe active logical nodes to standby.
 2. A method as claimed in claim 1,wherein the load in the network element is distributed between thecluster nodes that comprise active logical nodes.
 3. A method as claimedin claim 1, wherein the traffic in the network element is distributedbetween the cluster nodes that comprise logical nodes.
 4. A method asclaimed in claim 1, wherein the traffic in the network element isdistributed on the basis of a specific load allocation plan between thecluster nodes that comprise logical nodes.
 5. A method as claimed inclaim 1, wherein also an individual external routing address is definedfor each load allocation alternative on the basis of which data istransmitted to the network element.
 6. A method as claimed in claim 1,wherein information is further maintained on a primary and secondarycluster node associated with the load allocation alternative, wherebydata is transmitted to the primary cluster node and after a switchoverof a load allocation alternative, data is transmitted to the secondarycluster node of the load allocation alternative.
 7. A method as claimedin claim 1, wherein also performing a switchover of a load allocationalternative is performed such that after the switchover, data istransmitted through a physical interface of the backup cluster node tothe redundancy unit of the cluster node.
 8. A method as claimed in claim1, wherein the network element is backed up without a complete doublingof the number of the cluster nodes.
 9. A method as claimed in claim 1,wherein said logical nodes are software-associated components of thecluster nodes.
 10. A communications system comprising a network elementthat comprises at least a first and a second parallel physical clusternode, which cluster nodes are capable of transmitting data, whereby thefirst cluster node is configured to serve as a redundancy unit to thesecond cluster node and vice versa, wherein the system is configured tomaintain logical nodes at least in the first and second cluster node,form load allocation alternatives of the logical nodes such that thefirst logical node of the load allocation alternative resides in thefirst cluster node and the second logical node in the second clusternode, whereby the first logical node is active and the second on standbyor vice versa, and perform, when a cluster node malfunctions, aswitchover of the load allocation alternatives, the active logical nodesof which reside in the faulty cluster node, by changing their logicalnodes from standby to active and the active logical nodes to standby.11. A communications system as claimed in claim 10, wherein it isconfigured to distribute the load in the network element between thecluster nodes comprising active logical nodes.
 12. A communicationssystem as claimed in claim 10, wherein it is configured to define foreach load allocation alternative an individual external routing address,on the basis of which data is transmitted to the network element.
 13. Acommunications system as claimed in claim 10, wherein it is configuredto maintain information on a primary and secondary cluster nodeassociated with the load allocation alternative, whereby data istransmitted to the primary cluster node and after a switchover, data istransmitted to the secondary cluster node of the load allocationalternative.
 14. A communications system as claimed in claim 10, whereinit is configured to perform a switchover of the load allocationalternative in such a manner that after the switchover, data istransmitted through a physical interface of the backup cluster node tothe redundancy unit of the cluster node.
 15. A communications system asclaimed in claim 10, wherein the system is also configured to maintaininformation on the cluster node serving as the receiver of transmitteddata, through which the data is forwarded to the active cluster node,and to forward the data to the cluster node serving as the receiver. 16.A communications system as claimed in claim 10, wherein the system isalso configured to transmit data to one or more cluster nodes of thenetwork element separately, whereby they either reject the data orreceive it based on the routing address of the load allocationalternative.
 17. A communications system as claimed in claim 10, whereinthe system is also configured to transmit data to all cluster nodes ofthe network element in one go, whereby they either reject the data orreceive it based on the routing address of the load allocationalternative.
 18. A communications system as claimed in claim 10, whereinthe system is configured to back up the network element without acomplete doubling of the number of the cluster nodes.
 19. Acommunications system as claimed claim 10, wherein said logical nodesare software-associated components of the cluster nodes.
 20. A networkelement of a communications system, comprising at least a first and asecond parallel physical cluster node, which are capable of transmittingdata, whereby the first cluster node is a redundancy unit to the secondcluster node and vice versa, wherein the network element comprises afirst routine for maintaining logical nodes at least in the first andthe second cluster node, a second routine for forming load allocationalternatives of the logical nodes such that the first logical node ofthe load allocation alternative resides in the first cluster node andthe second logical node resides in the second cluster node, whereby thefirst logical node is active and the second on standby or vice versa,and a third routine for changing, when a cluster node malfunctions, theload allocation of the logical nodes of the load allocationalternatives, the active logical nodes of which reside in the faultycluster node, by changing the logical nodes from standby to active andthe active nodes to standby.
 21. A network element as claimed in claim20, wherein it comprises a fourth routine for distributing the load inthe network element between the cluster nodes that comprise activelogical nodes.
 22. A network element as claimed in claim 20, wherein italso comprises a sixth routine for distributing the traffic in thenetwork element between the cluster nodes that comprise logical nodes.23. A network element as claimed in claims 20, 21 or 22, characterizedin that claim 20, wherein it also comprises load allocation means asixth routine for distributing the traffic in the network element on thebasis of a specific load allocation plan between the cluster nodes thatcomprise logical nodes.
 24. A network element as claimed in claim 20,wherein it also comprises a seventh routine for defining an individualexternal routing address for each load allocation alternative, on thebasis of which routing address, data is transmitted to the networkelement.
 25. A network element as claimed in claim 20, wherein saidfirst routine is also arranged to maintain information on a primary anda secondary cluster node associated with the load allocationalternative, whereby data is transmitted to the primary cluster node andafter a switchover, data is transmitted to the secondary cluster node ofthe load allocation alternative.
 26. A network element as claimed inclaim 20, wherein it also comprises means an eighth routine for changingload allocation in such a manner that after the switchover of a loadallocation alternative, data is transmitted through a physical interfaceof the backup cluster node to the redundancy unit of the cluster node.27. A network element as claimed in claim 20, wherein it also comprisesa ninth routine for transmitting data by using said routing addressdefined for the load allocation alternative even after a switchover ofthe load allocation alternative.
 28. A network element as claimed inclaim 20, wherein it also comprises a tenth routine for performing aswitchover of a load allocation alternative inside the network element.29. A network element as claimed in claim 20, wherein it comprises aneleventh routine for backing up the network element without a completedoubling of the number of the cluster nodes.
 30. A network element asclaimed in claim 20, wherein said logical nodes are software-associatedcomponents of the cluster nodes.
 31. A network element as claimed inclaim 20, wherein it is a gateway GPRS support node of a GPRS system.32. A network element of a communications system, comprising at least afirst and a second parallel physical cluster node, which are capable oftransmitting data, whereby the first cluster node is a redundancy unitto the second cluster node and vice versa, wherein the network elementcomprises maintenance means for maintaining logical nodes at least inthe first and the second cluster node, first forming means for formingload allocation alternatives of the logical nodes such that the firstlogical node of the load allocation alternative resides in the firstcluster node and the second logical node resides in the second clusternode, whereby the first logical node is active and the second on standbyor vice versa, and execution means for changing, when a cluster nodemalfunctions, the load allocation of the logical nodes of the loadallocation alternatives, the active logical nodes of which reside in thefaulty cluster node, by changing the logical nodes from standby toactive and the active nodes to standby.
 33. A network element as claimedin claim 32, wherein it comprises load allocation means for distributingthe load in the network element between the cluster nodes that compriseactive logical nodes.
 34. A network element as claimed in claim 32,wherein it also comprises load allocation means for distributing thetraffic in the network element between the cluster nodes that compriselogical nodes.
 35. A network element as claimed in claims 32, wherein italso comprises load allocation means for distributing the traffic in thenetwork element on the basis of a specific load allocation plan betweenthe cluster nodes that comprise logical nodes.
 36. A network element asclaimed in claim 32, wherein it also comprises means for defining anindividual external routing address for each load allocationalternative, on the basis of which routing address, data is transmittedto the network element.
 37. A network element as claimed in claim 32,wherein said maintenance means are also arranged to maintain informationon a primary and a secondary cluster node associated with the loadallocation alternative, whereby data is transmitted to the primarycluster node and after a switchover, data is transmitted to thesecondary cluster node of the load allocation alternative.
 38. A networkelement as claimed in claim 32, wherein it also comprises means forchanging load allocation in such a manner that after the switchover of aload allocation alternative, data is transmitted through a physicalinterface of the backup cluster node to the redundancy unit of thecluster node.
 39. A network element as claimed in claim 32, wherein italso comprises switching means for transmitting data by using saidrouting address defined for the load allocation alternative even after aswitchover of the load allocation alternative.
 40. A network element asclaimed in claim 32, wherein it also comprises means for performing aswitchover of a load allocation alternative inside the network element.41. A network element as claimed in claim 32, wherein it comprises meansfor backing up the network element without a complete doubling of thenumber of the cluster nodes.
 42. A network element as claimed in claim32, wherein said logical nodes are software-associated components of thecluster nodes.
 43. A network element as claimed in claims 32, wherein itis a gateway GPRS support node of a GPRS system.